[systemd-devel] sd-bus example code for SetLinkDNS()

[Tilman Baumann]

Little follow-up question. What would you say is best practice for a vpn
client?

- Add commandline option --update-systemd-resolved or so
- Autodetect existence of the interface and just do it? (How?)
- Always try to do the update but silently ignore if it fails and fall
back to updating resolve.conf instead?

I was going with the commandline option. But I'm beginning to hate it.
Do the right thing if you know what's right rings better with me.

I will have to introduce a option for domain filter (and search domain)
anyway, that stuff will never work out of the box.
The VPN protocol is one of those nasty ppp over ssl jobs. And the only
fields passed down to the client are the nameservers.

Cheers
 Tilman

On 21.07.2017 14:35, Lennart Poettering wrote:
> On Wed, 19.07.17 11:20, Tilman Baumann (tilman at baumann.name) wrote:
> 
>> Hi folks,
>>
>> I'm trying to teach a vpn software (openfortivpn) how to properly set up
>> DNS in a systemd-resolve environment.
>>
>> I'm trying to set up a equivalent to this in C.
>> busctl call org.freedesktop.resolve1 /org/freedesktop/resolve1
>> org.freedesktop.resolve1.Manager SetLinkDNS 'ia(iay)' 16 2 2 4 10 10 10
>> 10 2 4 10 10 10 11
>> [https://gist.github.com/tbaumann/d484efb2e27613654a52dbe11cfe53b8]
>>
>> I came up with this quick proof of concept code based on the example
>> code in the sd-bus docu.
>> Of course it segfaults. No surprise, I have done nothing to hint at the
>> length of the inner byte array. (ay)
>>
>> I was unable to find any example code that would give me a hint on how
>> to pass such more complex data structures into sd_bus_call_method()
>>
>> int SetLinkDNSv4(sd_bus *bus, int if_index, struct in_addr ns1, struct
>> in_addr ns2) {
>>   sd_bus_error error = SD_BUS_ERROR_NULL;
>>   sd_bus_message *m = NULL;
>>   int r;
>>   struct dns_address {
>>     int sin_family;
>>     struct in_addr ip_addr;
>>   };
>>   struct dns_address addresses[2];
>>
>>
>>   addresses[0].sin_family = AF_INET;
>>   addresses[0].ip_addr = ns1;
>>   addresses[1].sin_family = AF_INET;
>>   addresses[1].ip_addr = ns2;
>>
>>   r = sd_bus_call_method(bus,
>>                          "org.freedesktop.resolve1",           /*
>> service to contact */
>>                          "/org/freedesktop/resolve1",          /* object
>> path */
>>                          "org.freedesktop.resolve1.Manager",   /*
>> interface name */
>>                          "SetLinkDNS",                         /* method
>> name */
>>                          &error,                               /* object
>> to return error in */
>>                          &m,                                   /* return
>> message on success */
>>                          "ia(iay)",                            /* input
>> signature */
>>                          if_index,
>>                          2,                                    /* Array
>> size */
>>                          addresses);
>> }
>>
>> [Full code:
>> https://gist.github.com/tbaumann/0f466c984c858767c966458d53483697]
>>
>> My guess is that I can have it easier if I somehow use
>> sd_bus_message_append() to assemble the message. But I don't see a clear
>> path either.
> 
> You have to do something like this:
> 
> sd_bus_message_new_method(..., &m);
> sd_bus_message_append(m, "i", ifindex);
> sd_bus_message_open_container(m, 'a', '(iay)');
> for (i = 0; i < n_addresses; i++) {
>         sd_bus_message_open_container(m, '(', "iay"));
>         sd_bus_message_append(m, "i", addresses[i].sin_family);
>         sd_bus_message_append_array(m, 'y', &addresses[i].ip_addr, sizeof(addresses[i].ip_addr));
>         sd_bus_message_close_container(m);
> }
> sd_bus_message_close_container(m);
> sd_bus_message_send(..., m);
> sd_bus_message_unref(m);
> 
> (not tested, just written down from the top of my head, and of course,
> don't forget to add proper error checking)
> 
> Lennart
>